It is well known that fluoropolymers have excellent chemical and heat resistance and in general are hydrophobic. It is also known that expanded porous polytetrafluoroethylene (ePTFE) polymers have superior strength properties. Thus, expanded porous polytetrafluoroethylene is useful as a filter media for organic solvents and for use in harsh chemical environments.
However, because of the hydrophobicity of fluoropolymers, aqueous dispersions cannot readily be filtered through filters made from these fluoropolymers. Such filters can be prewetted with organic solvents followed by flushing with water or using pressure to overcome the lack of affinity between the hydrophobic filter and the polar aqueous dispersion. However, such prewetting is expensive over the long term and can lead to "gas-lock" or "dewetting."
For these reasons, there have been various attempts to make fluoropolymer surfaces more hydrophilic and receptive to wetting with water while still maintaining their desirable properties. One approach is to coat the surface and the interior of the pores with a fluorinated surfactant to improve hydrophilicity. Since the fluorosurfactant is bound to the surface of the membrane only by means of chemical affinity, the weakness of this approach is that over a period of time the fluorosurfactant will be washed out by the aqueous medium and the membrane will lose its water-wettability. In an attempt to solve this problem, another approach has been to use a fluorosurfactant which is then crosslinked by an irradiation treatment using a high energy radiation beam such as Gamma ray, electron beam or non-equilibrium plasma. Such a crosslinked material will not diffuse out of the fluoropolymer matrix even when it is exposed to aqueous flow for an extended period of time. However, the high energy radiation weakens the mechanical strength of the fluoropolymer and the fluorinated surfactant will also suffer adverse effects ranging from deterioration of properties to alteration of its chemical properties.